In fiber optic systems, it is frequently desirable to remove, select, or suppress certain propagation modes of an optical signal. For example, in a two-mode optical fiber having an optical signal propagating therein in the first and second order spatial propagation modes, it is known that the light propagating in the second order mode can be "stripped" from the optical signal within the fiber by forming a segment of the fiber's length into a coil having a suitably selected coil diameter. The coil diameter is selected to perturb the second order mode without substantially affecting the first order mode, such that light propagating in the second order mode of the fiber enters the cladding of the optical fiber and escapes therefrom. The portion propagating in the first order mode is guided by the fiber and remains within the fiber's core. This light in the first order mode does not ordinarily couple to the second order mode due to the difference in propagation constants between the first order and second order modes of the fiber. Thus, after passing through the length of coil fiber, the optical signal propagates substantially solely in the first order propagation mode. Thus, the "mode stripper" allows the selection of the light propagating in the first order mode through removal of the light in the second order mode.
Although a mode stripper is an advantageous device, its uses are limited, since the light that is stripped from the fiber merely radiates into free space. The mode stripper does not have the capability of coupling selected modes of two different optical fibers. The ability to couple a selected spatial mode of one optical fiber to a selected spatial mode of another optical fiber has many uses in a fiber optic system. For example, in a fiber optic frequency shifter wherein light propagating in a first order mode is coupled to a second order mode and is shifted in frequency, it is desirable that the portion of the frequency shifted light in the second order mode be coupled to a selected mode of a different fiber so as to be able to transmit the light at the shifted frequency. Thus, a need exists for a mode selector which can, for example, separate lower order modes (e.g., a first order mode) from higher order modes (e.g., a second order mode) and provide the higher order mode as an output for further use in the fiber optic system.